Soil erosion is very costly. It has been estimated that in the U.S., wind and water erosion causes $17 billion in on-site damages through lost soil and nutrients. It is estimated that off-site damages from siltation, drainage, flooding and undermining of existing structures costs the U.S. $27 billion dollars each year. The erosion control industry has grown rapidly to meet these demands. Highway and street construction is a good indicator for demand of erosion control services. In 2008, the value of construction manufacturing rose 52% from 2007, compared to the private sector construction trend that was down 10% in the first period.
The presence of plant life reduces erosion by slowing run-off of water, allowing it to be absorbed into the soil. Plant root systems further stabilize the soil by trapping soil between the roots, preventing it from being carried away with flowing water. Erosion control is an environmental consideration any place where erosion occurs. It is common in construction areas where new slope embankments and channels are created. When vegetation has been removed to prepare for construction or new drainage systems, water tends to run off the land rather than be absorbed by it. Much of the soil can be carried away with the run-off, especially where the soil is primarily sand. When the topsoil has washed away, it is more difficult for seeds to germinate and become established in poorly structured soil.
It is known to add a layer of straw or wood chips to hold new seed in place and thereby help control erosion. Although the weight of the straw and wood assist in holding down the seed, heavy rain or run-off can carry them away, particularly on steep slopes. One reason this occurs is because the straw and wood are loose on the slope, and they can become dislodged and entrained in the water flow. Heavy run-off also washes away the straw and wood products, eventually leaving no protection at all. Even if they stay in place when organic materials degrade, such as wheat straw, they absorb nitrogen and other plant nutrients from the soil. Fertility of the soil is thereby reduced, inhibiting germination and growth of plants, especially the roots. This delays forming a root system to trap the soil, holding it in place after the organic mat biodegrades.
To hold the seed in place, erosion-control matrices have been developed that are surface-applied to the soil and are unlikely to wash away. Two types of erosion-control matrices are commonly used, a rolled-out matrix and a spray matrix. The rolled-out matrix is brought to the job site as a ready-to-use blanket. It is unrolled and stapled to the soil. Unrolling the blanket, assuring overlap with adjoining sections of blanket and affixing it to the soil is a labor-intensive process that can be very expensive. A spray matrix is formed in situ by spraying a liquid or slurry onto the soil that sets, hardens or dries to cover seed that has been distributed.
Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Dihydrate synthetic gypsum is a byproduct of flue gas desulfurization processes from power plants. Plaster of Paris is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or calcium sulfate hemihydrate. When it is mined, raw gypsum is found in the dihydrate form. In this form, there are approximately two water molecules of water associated with each molecule of calcium sulfate. In order to produce the hemihydrate form, the gypsum is calcined to drive off some of the water of hydration by the following reaction:CaSO4.2H2O+Heat→CaSO4.½H2O+ 3/2H2O
Calcium sulfate hemihydrate can produce at least two crystal forms. Alpha-calcined gypsum is made by a continuous process or a lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated crystals are characteristic of the beta-calcined gypsum. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained and the cost is relatively low compared to the alpha-calcined gypsum.
A number of useful gypsum products can be made by mixing the calcium sulfate hemihydrate with water and shaping the resulting product slurry into the desired shape. The product slurry is permitted to set by allowing the calcium sulfate hemihydrate to react with sufficient water to convert the hemihydrate into a matrix of interlocking dihydrate crystals. As the gypsum matrix forms, the product slurry becomes firm and holds the desired shape. Excess water must then be removed from the product by drying.
Dihydrate gypsum is known for use as a soil conditioner. When added to the soil, it adds calcium, a mineral that is utilized by plants. It also breaks down soils that contain clays. The dense structure of clay delays establishment of new plants by making it difficult for the roots to penetrate the soil. Because of gypsum's ability to provide soil nutrients, spray matrices using gypsum have been tried with limited success in sandy soils.
Erosion-control matrices are known in the marketplace that are gypsum-based. They are spray-applied to the soil with traditional hydraulic seeding equipment. One product, marketed as AIRTROL® Geobinder (USG Corp., Chicago, Ill.) is a pure slurry of calcined gypsum in water. A second product, ENVIRO-SHIELD® Bonded Fiber Matrix (USG Corp., Chicago, Ill.) includes mulch and a polymer in the gypsum slurry. These products retain clay soils while awaiting development of a root system from vegetation; however, these products are not adequately effective in preventing erosion of sandy soils.